Update links for courses
Update 2000-01-01-concepts.markdown

Rearranged content to make it more explanatory
Update 2000-01-01-concepts.markdown

Minor grammar and syntax changes.
Update 2000-01-01-concepts.markdown

Text changes, layout changes
Update 2000-01-01-concepts.markdown

Text updates
Merge remote-tracking branch 'upstream/gh-pages' into gh-pages
removing console.log